Consider the method of providing power and signal flow to and from a sonde supported in a well bore on a logging cable. A logging cable having only two wires is known as a monocable. In the typical arrangement, it not only physically supports the sonde to be raised or lowered in the well, but it also provides two wire transmission for power and signal. Power is transmitted from a power source at the wellhead to a power load in the sonde. The power load is a device which consumes power while also forming an output signal from the operation of the logging devices within the sonde. The precise nature of the logging device is not particularly critical. An example is logging equipment which provides SP measurements which are relayed up to the surface. The data is supplied to data conversion circuitry which terminates in a telemetry transmitter to enable the data to be transferred up the logging cable. The two wire monocable must therefore transmit power from the power source at the wellhead along the cable and must also transmit the data of interest along the cable.
Customarily, power is furnished at 400 hertz. This enables the use of transformers to provide various voltages in the sonde. Equally, AC power is preferable over DC power because there is a risk of magnetizing the spooled cable on the storage reel supported on the logging truck at the surface. Moreover, the use of AC power enables filters to isolate the telemetry signal because it is ordinarily transmitted at a frequency and bandwidth enabling separation from the power transmission equipment.
While not all wells are extremely deep, it is not uncommon for a well to be over 20,000 feet deep. Obviously, the logging cable must exceed the length of the well. Logging cables are as long as 25,000 or 30,000 feet. A fairly substantial driving signal from the telemetry transmitter carried in the sonde is required to transmit a data pulse through the long monocable. Perhaps the difficulty of such data transmission is better understood by reviewing the typical circuit presently in use. As will be noted with regard to FIG. 2 of the drawings herein, a blocking capacitor 30 in the sonde isolates the AC power intended for the power load from the telemetry driver circuitry. FIG. 2 further incorporates a series inductance 31 cooperating with a series capacitor 32 for furnishing power to the power load. This passes the AC supply power and blocks the high frequency telemetry signal. As will be observed in FIG. 2, the quiescent state finds the telemetry driver transistors 33 and 34 non-conducting, and the transistor resistor 35 is transformer coupled through the transformer 36 into the circuit in series with the capacitor 30. In this quiescent condition, the supply frequency is imposed across the transformer 36 in an amount depending upon the ratio of capacitor 30 impedance and the transformed impedance of resistor 35. There is a minimum value of resistor 35 for this circuit because, when one of the transistors 33 or 34 begins to conduct, the resistor 35 causes a current to be drawn through the output transformer 36 in a bucking direction. For example, if the resistor 35 were zero ohms, then there would be no output signal developed due to opposite currents in the two halves of the primary. However, if resistor 35 is large, then the voltage across the transformer will cause the telemetry pulses to be amplitude modulated, thus the circuit causes a compromise in design parameters which results in less than optimum telemetry transfer characteristics. This occurs because the quiescent voltage across transformer 36 causes the output pulse to be amplitude modulated.
The present invention offers an improvement over the foregoing. The low impedance output in the quiescent state means that the output pulses from the multiplexed data signal are not peak amplitude modulated. Rather, the isolation which is achieved enables the apparatus to successfully telemeter the more sensitive data signal on the monocable without pulse amplitude modulation or other undesired effects of shared transmission along the monocable.
An additional advantage of the present apparatus is that a low impedance at the output simplifies power filtering. The low impedance helps to shunt high frequency components which may be generated by the power supply to ground. The circuitry also utilizes less power. Moreover, the output stage of the apparatus of the present invention does not exhibit thermal runaway characteristics at elevated temperatures. These advantages and features will be more readily applied upon a review of the detailed disclosure set forth below. Briefly, this apparatus is directed to a telemetry driver stage for use on a monocable where the cable shares AC power transmission.